you can buy a semi permanent purple hair dye such as one by the brands crazy colour, manic panic or directions. sold online in some alternative shops and possible claires accessories. don't pick a pale lilac pick a strong bold purple and add about a tsp to your conditioner to make a dark lilac colour. make sure it is totally evenly mixed!!! use this as a mask for 10-20 mins.
its trial and error regards quantities added and length of application so start small.
really works wonders though!